Given, the frequency of string $A$ is $f_A = 530 \text{ Hz}$. The beat frequency produced by strings $A$ and $B$ is $6 \text{ Hz}$. Therefore, the original frequency of string $B$ can be either: $f_B = f_A + 6 = 536 \text{ Hz}$ or $f_B = f_A - 6 = 524 \text{ Hz}$

The frequency of a stretched string is directly proportional to the square root of its tension ($f \propto \sqrt{T}$). When the tension in string $B$ is decreased, its frequency $f_B$ decreases.

Now, let's analyze both cases with the new decreased frequency $f_B'$: Case 1: If $f_B$ was $536 \text{ Hz}$, a decrease in tension would lower the frequency (e.g., to $535 \text{ Hz}$). The new beat frequency would be $|530 - 535| = 5 \text{ Hz}$. Here, the beat frequency decreases, which contradicts the given information. Case 2: If $f_B$ was $524 \text{ Hz}$, a decrease in tension would lower the frequency (e.g., to $523 \text{ Hz}$). The new beat frequency would be $|530 - 523| = 7 \text{ Hz}$. Here, the beat frequency increases to $7 \text{ Hz}$, which exactly matches the given condition.

Therefore, the original frequency of string $B$ must be $524 \text{ Hz}$.